We are using murine 3T3-L1 fat cells for studying hormonal regulation of synthesis, activation and transport of lipoprotein lipase (LPL) in cells. We have a radioimmunoassay for measuring LPL protein in cells and medium. We developed recently a method using radioactive sulfur for measuring synthesis of LPL. Earlier findings indicated that insulin increased the amounts of both LPL-immunoreactive protein and LPL activity in cell. Our recent findings establish that insulin stimulates synthesis of LPL. They also show that insulin has a faster and larger stimulatory effect on LPL activity than on LPL synthesis, which suggests that there is an inactive form of LPL in adipocytes and that insulin may have an important role in its being activated. Our recent findings also include evidence that insulin may be involved in translocation of LPL in cultured adipocytes, to sites where the enzyme is accessible to heparin in the medium. Preliminary findings using a radioimmunoassay for murine LPL suggest that normal amounts of LPL are present in tissues of mice with a lethal genetic absence of lipoprotein lipase activity in their tissues. Whether the deficiency is due to impaired synthesis or activation of LPL, or both, is under investigation.